Microscope objective

ABSTRACT

A microscope objective lens system comprising a plurality of lenses, at least one of the lenses being a graded refractive index lens (GRIN lens) whose refractive index varies as the radial distance from the optical axis varies, which is simpler in construction and in which aberrations are corrected sufficiently to a wide range of NA.

This is a continuation-in-part applicaton of Ser. No. 893,172 filed Aug.5, 1986, which was abandoned upon the filing hereof.

BACKGROUND OF THE INVENTION

(a) Field of the Invention

The present invention relates to a microscope objective lens system.

(b) Description of the Prior Art

It is necessary that various aberrations be sufficiently well-correctedin a microscope objective lens system. But, at the same time, it isnecessary to satisfy the requirements which limit the freedom of thecorrection of aberrations. For example, in order to obtain highresolution power, it is necessary for the numerical aperture (NA) to belarge. In order to avoid a damaging contact between an object and anobjective lens system, it is necessary for the working distance (WD) tobe large. In order to set the various kinds of objective lens systems onthe same revolver and to change them for use, it is necessary for atotal length of the lens system to be within predetermined limits.Moreover, in order to keep the optical tube length constant, it isnecessary for the distance between the rear focal point of an objectivelens system and the image surface to be constant independently ofmagnification.

It is very difficult to correct various aberrations sufficiently giventhese various limitations plus the limitation that the objective lenssystem is a magnifying system.

In order to overcome these difficulties, the conventional microscopeobjective lens system cannot but be an extremely complicated opticalsystem which comprises a number of lenses having strong curvatures.

In order to simplify construction, there have been developed microscopeobjective lens systems in which aspherical lenses or graded refractiveindex lenses (GRIN lenses) are introduced. The examples where a GRINlens is applied to a microscope objective lens system are disclosed inJapanese Published Examined Patent Application Nos. 28057/72 and37405/82. The former only discloses the use of a GRIN lens in theimage-forming system of a microscope, but does not disclose anythingrelated to the correction of aberrations. On the other hand, in thelatter, a GRIN lens whose refractive index is expressed by theexponential function as described below is arranged, in the opticalsystem of a microscope, at the position where the height of the axialmarginal ray is smaller than that of the off-axial ray, so thatoff-axial aberrations are corrected.

    n=n.sub.0 e.sup.kr

where n₀ represents the refractive index at the center of the lens, k isthe parameter representing the gradient of the refractive index and rrepresents the radial distance from the optical axis.

In the case that the refractive index varies in accordance with theabove-mentioned function, it varies extraordinarily where r is large.Therefore, when the above-mentioned GRIN lens is arranged at theabove-mentioned position, it will be possible to correct only off-axialaberrations without any influence on axial aberration.

In a usual microscope objective lens system, however, when NA becomeslarge, axial light bundle and off-axial light bundle propagate almostoverlappingly so that the position where both the bundles separate fromeach other is quite close to the image-forming position. In theconventional construction, therefore, it is necessary to set a GRIN lensseparately and behind an objective lens. However, the position throughwhich a ray passes is different in each objective lens system, so thatwhen various kinds of objective lens systems are changed while the GRINlens is fixed, it will be impossible to always well-correct aberrations.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a microscopeobjective lens system which comprises, as a constructional element, agraded refractive index lens (GRIN lens) whose refractive index variesas the radial distance from the optical axis varies and in which NA islarge though various aberrations are corrected favorably.

It is necessary that in a microscope objective lens system sphericalaberration, coma, curvature of field and astigmatism etc. are correctedsufficiently. Once the power distribution is determined in order tocorrect curvature of field among these various aberrations, thelimitations for the other aberrations will become severe, so that itwill become difficult to correct all the aberrations favorably.

With an objective lens system which is divided into a front group whichmainly contributes to the enlargement of magnification or in whichentrance NA and exit NA are different largely from each other, and arear group which relays the exit ray from the front group to thepredetermined image-forming position (including ∞), in order to correctall of above-mentioned various aberrations sufficiently, it is necessaryfor the front group to have a correction of curvature of field as wellas suppression of the generation of spherical aberration and coma, andto have a strong refracting power.

In order to minimize spherical aberration, etc. to be produced in thefront group, however, it is desirable to adopt an aplanatic lenscomposition for the front group. In order to flatten the image surface,on the other hand, the objective lens system must satisfy Petzval'scondition, thereby making it necessary to modify to certain degrees theaplanatic composition for the front group. In such a case, sphericalaberration, etc. will remain in the front group.

As a method to solve this problem, it is effective to arrange a radialGRIN lens in the front group.

Petzval's sum of a radial GRIN lens PS is expressed by the followingformula:

    PS=Σφs/n.sub.0 +φG/n.sub.0.sup.2

wherein the reference symbols are defined as follows:

φs: refracting power on the radial GRIN lens surfaces

φG: refracting power of the radial GRIN lens medium

n₀ : refractive index on the optical axis of the radial GRIN lens

As is understood from the above-mentioned formula, Petzval's sum PS islittle dependent on the refracting power of medium φG. Therefore,Petzval's sum can easily be satisfied by arranging a radial GRIN lens inthe front group while maintaining an aplanatic composition of the frontgroup.

As another method to solve the above-mentioned problem, it isconceivable to adopt a radial GRIN lens in the rear group to enhanceaberration correcting function of the rear group.

The rays emitted from an object are gradually diverged while changing NAin the front group. Therefore, the off-axial upper ray and lower ray arefarther apart from each other as they are farther from the object, andremarkable asymmetrical aberration is produced in the front group. Thisaberration can be corrected by arranging a radial GRIN lens in the reargroup. Further, in the objective lens system having a large NA, theparaxial ray is widerly diverged also in the rear group and broadlyoverlapped with the off-axial ray. Therefore, the paraxial aberrationcan also be corrected by a radial GRIN lens arranged in the rear group.

By applying both the methods described above, i.e., adopting radial GRINlenses in both the front and rear groups, it is possible to obtain ahigher effect as a multiplicative effect of both the methods or design amicroscope objective lens system in which aberrations are corrected morefavorably.

In the foregoing descriptions, the front and rear groups are divideddepending on magnification levels. In the widely used microscopeobjective lens systems such as Amici type and Petzval's type, however,it is possible to regard the intersection between the off-axialprincipal ray and the optical axis as the dividing point, i.e., toregard the lens group located on the object side from the intersectionas a frong group and the lens group located on the image side as a reargroup.

The so-called plan objective lens system for which flatness of the imagesurface is strongly demanded is mostly designed as a lens systemcomprising a meniscus lens having a strongly concave surface on theextreme object side or a Gauss type in order to correct curvature offield.

The lens system comprising a meniscus lens on the extreme object side ispreferable for correction of aberration but does not permit prolongingthe working distance. It is therefore desirable to select a planesurface in place of the concave surface. When selecting such a planesurface, however, refractive power is generally too strong to correctPetzval's sum. When this lens is replaced with a radial GRIN lens havingnegative refracting power, it is possible to maintain flatness of theimage surface owning to the Petzval's term (φG/n₀ ²) of medium even ifthe surface on the entrance side is plane.

In a case where the Gauss type is adopted for the microscope objectivelens system according to the present invention, on the other hand, aconcave surface is arranged at a location apart from the surface on theextreme object side and the surface arranged on the extreme object sideis not a concave surface having strong curvature. Therefore, the Gausstype of objective lens system permits reserving a relatively longworking distance. Taking workability, etc. into consideration, however,it is desirable to select a plane surface on the extreme object side.

When a plane surface is arranged on the extreme object side, thecomposition of the lens system deviates from the aplanatic composition,thereby aggravating spherical aberration and degrading balance ofaberrations. In order to correct this spherical aberration, it iseffective to arrange a radial GRIN lens on the rear side of the lenscomponent arranged on the extreme object side. A very high effect isobtained for correcting spherical aberration especially by adopting aradial GRIN lens as a lens component having a function to diverge theconverged paraxial ray.

In a case where a radial GRIN lens having a plane surface on the objectside is arranged on the extreme object side, too high a refractive indexgradient of the GRIN lens is favorable for correction of Petzval's sumbut allows positive spherical aberration to be producted. Accordingly,it is possible to obtain a microscope objective lens system having asimple composition and correcting aberrations with good balance byarranging a radial GRIN lens at a location, on the image side of theabove-mentioned lens component arranged on the extreme object side, inthe vicinity of widest divergence point of the paraxial ray or on theimage side thereof.

In the Embodiments described later, the following GRIN lens is used.That is, the refractive index distribution is expressed by the followingformula wherein the refractive index on the optical axis is given by n₀and the radial distance from the optical axis is given by r.

    n=n.sub.0 +n.sub.1 r.sup.2 +n.sub.2 r4+n.sub.3 r6+ . . .

where n₁, n₂, n₃, . . . respectively represent the 2nd-, 4th-, 6th-, . .. order coefficients.

In order to have a GRIN lens possess the full power of the correction ofaberrations, when a GRIN lens is arranged in the front group, it will bedesirable to satisfy the following condition (1):

    |n.sub.1 |>7×10.sup.-4 /f.sup.2    ( 1)

When a GRIN lens is arranged in the rear group, it will be desirable tosatisfy the following condition (2):

    |n.sub.1 |>1×10.sup.-4 /f.sup.2    ( 2)

If the conditions (1) or (2) are not satisfied, the difference betweenthe refractive index at the center and that at the periphery will becometoo small, and it will be difficult to correct aberrations sufficiently.

If the variation of the high-order refractive index becomes too large,it will become difficult to keep the balance between the aberration bythe ray passing near the optical axis and that by the ray passingthrough the periphery of the lens. In order to avoid this, it will bedesirable to satisfy the following conditon (3) wherein n_(i) representsthe value of each order coefficient n_(i) normalized by the focal lengthof the total system or n_(i) =(n_(i) /f^(2i)):

    |n.sub.1 |>|n.sub.i |, i≧3 (3)

Especially, it will be desirable to make the difference no less than oneor two orders between the 2nd-order coefficient and each of thehigher-order coefficient in order to correct aberrations with goodbalance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 14 respectively show the lens constructions ofEmbodiments 1 through 14 of the microscope objective lens systemaccording to the present invention; and

FIGS. 15 through 28 show graphs illustrating aberration curves ofEmbodiments 1 through 14 of the microscope objective lens systemaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFFERED EMBODIMENTS

Preferred Embodiments of the microscope objective lens system accordingto the present invention are shown below:

    ______________________________________                                        Embodiment 1                                                                  r.sub.1 = -0.6144                                                                            d.sub.1 = 1.0054                                                                          n.sub.01 = 1.78590*                                r.sub.2 = -0.9063                                                                            d.sub.2 = 0.0129                                               r.sub.3 = -11.8711                                                                           d.sub.3 = 0.5453                                                                          n.sub.02 = 1.76180*                                r.sub.4 = -1.9559                                                                            d.sub.4 = 2.5506                                               r.sub.5 = 14.2613                                                                            d.sub.5 = 0.2171                                                                          n.sub.03 = 1.49700                                 r.sub.6 = -2.8644                                                                            d.sub.6 = 0.0129                                               r.sub.7 = 2.5836                                                                             d.sub.7 = 0.6761                                                                          n.sub.04 = 1.65830                                 r.sub.8 = ∞                                                                            d.sub.8 = 0.3728                                                                          n.sub.05 = 1.61340                                 r.sub.9 = 1.1081                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             0.16838        0.60574 × 10.sup.-1                           the 2nd lens                                                                             -0.13761       -0.54513 × 10.sup.-2                          ______________________________________                                        f = 1               WD = 0.2456                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 2                                                                  r.sub.1 = -0.7070                                                                            d.sub.1 = 1.0402                                                                          n.sub.01 = 1.78590*                                r.sub.2 = -0.9457                                                                            d.sub.2 = 0.0130                                               r.sub.3 = -7.5963                                                                            d.sub.3 = 0.5582                                                                          n.sub.02 = 1.76180*                                r.sub.4 = -1.7635                                                                            d.sub.4 = 2.2760                                               r.sub.5 = 7.1126                                                                             d.sub.5 = 0.4596                                                                          n.sub.03 = 1.49700                                 r.sub.6 = -2.7181                                                                            d.sub.6 = 0.0130                                               r.sub.7 = 2.7615                                                                             d.sub.7 = 1.1005                                                                          n.sub.04 = 1.65830                                 r.sub.8 = 1.0582                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             0.28629        0.10903                                             the 2nd lens                                                                             -0.18390       -0.78497 × 10.sup.-2                          ______________________________________                                        f = 1               WD = 0.2486                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 3                                                                  r.sub.1  = -0.4538                                                                           d.sub.1 = 0.6875                                                                          n.sub.01 = 1.83400                                 r.sub.2 = -0.7384                                                                            d.sub.2 = 0.0128                                               r.sub.3 = -2.2225                                                                            d.sub.3 = 0.2726                                                                          n.sub.02 = 1.49700                                 r.sub.4 = -1.1462                                                                            d.sub.4 = 0.0128                                               r.sub.5 = -50.2670                                                                           d.sub.5 = 0.6930                                                                          n.sub.03 = 1.76180                                 r.sub.6 = -3.5196                                                                            d.sub.6 = 2.4305                                               r.sub.7 = 3.5395                                                                             d.sub.7 = 0.3499                                                                          n.sub.04 = 1.49700                                 r.sub.8 = -4.1378                                                                            d.sub.8 = 0.0128                                               r.sub.9 = 1.9530                                                                             d.sub.9 = 0.8743                                                                          n.sub.05 = 1.65830*                                r.sub.10 = 0.9857                                                             ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 5th lens                                                                             -0.14689 × 10.sup.-2                                                                   -0.60052 × 10.sup.-3                          f = 1               WD = 0.2434                                               m =  20 x           NA = 0.46                                                 ______________________________________                                        Embodiment 4                                                                  r.sub.1 = -0.4496                                                                            d.sub.1 = 0.6812                                                                          n.sub.01 = 1.83400                                 r.sub.2 = -0.7511                                                                            d.sub.2 = 0.0127                                               r.sub.3 = -2.6794                                                                            d.sub.3 = 0.2542                                                                          n.sub.02 = 1.49700                                 r.sub.4 = -1.0686                                                                            d.sub.4 = 0.0127                                               r.sub.5 = -39.6904                                                                           d.sub.5 = 0.6420                                                                          n.sub.03 = 1.76180                                 r.sub.6 = -4.0568                                                                            d.sub.6 = 1.4876                                               r.sub.7 = 4.1051                                                                             d.sub.7 = 0.3941                                                                          n.sub.04 = 1.60178*                                r.sub.8 = -4.6364                                                                            d.sub.8 = 0.0127                                               r.sub.9 = 1.9945                                                                             d.sub.9 = 0.8333                                                                          n.sub.05 = 1.66083*                                r.sub.10 = 0.9754                                                             ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 4th lens                                                                             -0.36453 × 10.sup.-1                                                                   0.98488 ×  10.sup.-2                          the 5th lens                                                                             0.13955 × 10.sup.-1                                                                    -0.25987 × 10.sup.-1                          ______________________________________                                        f = 1               WD = 0.2427                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 5                                                                  r.sub.1 = -0.6202                                                                            d.sub.1 = 1.0101                                                                          n.sub.01 = 1.78590*                                r.sub.2 = -0.9334                                                                            d.sub.2 = 0.0129                                               r.sub.3 = -25.9860                                                                           d.sub.3 = 0.5460                                                                          n.sub.02 = 1.76180*                                r.sub.4 = -1.8355                                                                            d.sub.4 = 2.5837                                               r.sub.5 = 17.0878                                                                            d.sub.5 = 0.2177                                                                          n.sub.03 = 1.49700                                 r.sub.6 = -2.8475                                                                            d.sub.6 = 0.0129                                               r.sub.7 = 2.3126                                                                             d.sub.7 = 1.0278                                                                          n.sub.04 = 1.65380*                                r.sub.8 = 1.3126                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             0.20954        0.55868 × 10.sup.-1                           the 2nd lens                                                                             -0.12844       0.81108 × 10.sup.-2                           the 4th lens                                                                             0.18233 × 10.sup.-2                                                                    0.56092 × 10.sup.-3                           ______________________________________                                        f = 1               WD = 0.2464                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 6                                                                  r.sub.1 = -0.6082                                                                            d.sub.1 = 0.9974                                                                          n.sub.01 = 1.77250*                                r.sub.2 = -0.9962                                                                            d.sub.2 = 0.0127                                               r.sub.3 = 4.5931                                                                             d.sub.3 = 0.5346                                                                          n.sub.02 = 1.77250*                                r.sub.4 = -2.1599                                                                            d.sub.4 = 2.6874                                               r.sub.5 = 5.1919                                                                             d.sub.5 = 1.0629                                                                          n.sub.03 = 1.83400*                                r.sub.6 = 1.5327                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             0.15622        0.27687 × 10.sup.-1                           the 2nd lens                                                                             -0.93618 × 10.sup.-1                                                                   0.19220 × 10.sup.-1                           the 3rd lens                                                                             -0.16188       0.32660 × 10.sup.-1                           ______________________________________                                        f = 1               WD =  0.2417                                              m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 7                                                                  r.sub.1 = -0.5249                                                                        d.sub.1 = 0.9910                                                                              n.sub.01 = 1.77250                                 r.sub.2 = -1.0014                                                                        d.sub.2 = 0.0124                                                   r.sub.3 = 4.7627                                                                         d.sub.3 = 0.5335                                                                              n.sub.02 = 1.77250*                                r.sub.4 = -2.2014                                                                        d.sub.4 = 2.6257                                                   r.sub.5 = -23.3838                                                                       d.sub.5 = 1.0309                                                                              n.sub.03 = 1.83400*                                r.sub.6 = 3.3579                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 2nd lens                                                                             -0.55298 × 10.sup.-1                                                                   -0.63272 × 10.sup.-2                          the 3rd lens                                                                             -0.13406       0.43943 × 10.sup.-1                           ______________________________________                                        f = 1               WD = 0.2371                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 8                                                                  r.sub.1 = ∞                                                                            d.sub.1 = 0.8889                                                                          n.sub.01 =  1.63980*                               r.sub.2 = -7.4105                                                                            d.sub.2 = 0.0131                                               r.sub.3 = 1713.8486                                                                          d.sub.3 = 0.2502                                                                          n.sub.02 = 1.49700                                 r.sub.4 = -1.7788                                                                            d.sub.4 = 0.5388                                                                          n.sub.03 = 1.79952*                                r.sub.5 = -1.3234                                                                            d.sub.5 = 0.0131                                               r.sub.6 = 24.5284                                                                            d.sub.6 = 0.4578                                                                          n.sub.04 = 1.67000*                                r.sub.7 = -3.7949                                                                            d.sub.7 = 1.9044                                               r.sub.8 = 2.6069                                                                             d.sub.8 = 0.6026                                                                          n.sub.05 = 1.65830*                                r.sub.9 = ∞                                                                            d.sub.9 = 0.6903                                                                          n.sub.06 = 1.61293*                                r.sub.10 = 0.6670                                                                        n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             0.56264        -0.51213                                            the 3rd lens                                                                             -0.11046       0.37325 × 10.sup.-1                           the 4th lens                                                                             -0.12854       -0.73600 × 10.sup.-1                          the 5th lens                                                                             -0.25773       -0.72271 × 10.sup.-1                          the 6th lens                                                                             0.16576 × 10.sup.-2                                                                    -0.12224                                            ______________________________________                                        f = 1               WD = 0.2496                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 9                                                                  r.sub.1 = ∞                                                                            d.sub.1 = 0.5027                                                                          n.sub.01 = 1.83400*                                r.sub.2 = -0.7363                                                                            d.sub.2 = 0.0125                                               r.sub.3 = 0.5038                                                                             d.sub.3 = 0.2311                                                                          n.sub.02 = 1.49700                                 r.sub.4 = ∞                                                                            d.sub.4 = 0.1868                                                                          n.sub.03 = 1.74000*                                r.sub.5 = 0.3873                                                                             d.sub.5 = 0.2353                                               r.sub.6 = -0.3567                                                                            d.sub.6 = 0.1005                                                                          n.sub.04 = 1.67270*                                r.sub.7 = ∞                                                                            d.sub.7 = 0.4094                                                                          n.sub.05 = 1.49700*                                r.sub.8 = -0.7164                                                                            d.sub.8 = 0.0184                                               r.sub.9 = 4.7830                                                                             d.sub.9 = 0.3580                                                                          n.sub.06 = 1.66755*                                r.sub.10 =  -1.2326                                                                          d.sub.10 = 1.5374                                              r.sub.11 = 1.0611                                                                            d.sub.11 = 0.6214                                                                         n.sub.07 = 1.54869                                 r.sub.12 = 0.7628                                                                        n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             -0.24460       -0.25363                                            the 3rd lens                                                                             0.36245 × 10.sup.-1                                                                    0.48691                                             the 4th lens                                                                             0.92425 × 10.sup.-1                                                                    0.63217                                             the 5th lens                                                                             0.10171        0.75023 × 10.sup.-1                           the 6th lens                                                                             -0.58875 × 10.sup.-1                                                                   -0.22649 × 10.sup.-2                          ______________________________________                                        f = 1               WD = 0.4513                                               m = 20 x            NA = 0.46                                                 ______________________________________                                        Embodiment 10                                                                 r.sub.1 = -0.8938                                                                            d.sub.1 = 1.3014                                                                          n.sub.01 = 1.60311*                                r.sub.2 = -1.0761                                                                            d.sub.2 = 0.0485                                               r.sub.3 = -12.5422                                                                           d.sub.3 = 1.2779                                                                          n.sub.02 = 1.49700*                                r.sub.4 = -2.1073                                                                            d.sub.4 = 0.1196                                               r.sub.5 =  -5.9203                                                                           d.sub.5 = 4.0147                                                                          n.sub.03 = 1.65830*                                r.sub.6 = -15.8260                                                                           d.sub.6 = 2.4285                                               r.sub.7 = 50.5226                                                                            d.sub.7 = 1.2080                                                                          n.sub.04 = 1.69680                                 r.sub.8 = -45.6551                                                                           d.sub.8 = 0.8486                                               r.sub.9 = 8.8504                                                                             d.sub.9 = 3.9661                                                                          n.sub.05 = 1.64769*                                r.sub.10 = 8.1084                                                                            d.sub.10 = 0.0901                                              r.sub.11 = 5.3751                                                                            d.sub.11 = 3.3167                                                                         n.sub.06 = 1.69680*                                r.sub.12 = 1.8122                                                             ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             -0.55383 × 10.sup.-1                                                                   0.75335 × 10.sup.-2                           the 2nd lens                                                                             0.36158 × 10.sup.-2                                                                    -0.67351 × 10.sup.-2                          the 3rd lens                                                                             -0.48852 × 10.sup.-2                                                                   0.58867 × 10.sup.-3                           the 5th lens                                                                             -0.91403 × 10.sup.-2                                                                   0.61851 × 10.sup.-4                           the 6th lens                                                                             0.29971 ×  10.sup.-2                                                                   -0.79363 × 10.sup.-3                          ______________________________________                                        f = 1               WD = 0.2747                                               m = 100 x           NA = 0.93                                                 ______________________________________                                        Embodiment 11                                                                 r.sub.1 = -0.9046                                                                            d.sub.1 = 1.3174                                                                          n.sub.01 = 1.60311*                                r.sub.2 = -1.0892                                                                            d.sub.2 = 0.0490                                               r.sub.3 = -14.5015                                                                           d.sub.3 = 1.2957                                                                          n.sub.02 = 1.49700*                                r.sub.4 = -2.1711                                                                            d.sub.4 = 0.1210                                               r.sub.5 = -9.4617                                                                            d.sub.5 = 4.1552                                                                          n.sub.03 = 1.65830*                                r.sub.6 = -74.3330                                                                           d.sub.6 = 2.6622                                               r.sub.7 = 14.4729                                                                            d.sub.7 = 1.2227                                                                          n.sub.04 = 1.69680                                 r.sub.8 = 76.2347                                                                            d.sub.8 = 1.4766                                               r.sub.9 = 7.4458                                                                             d.sub.9 = 4.4476                                                                          n.sub.05 = 1.64769*                                r.sub.10 = 5.3166                                                                            d.sub.10 = 0.0912                                              r.sub.11 = 4.1294                                                                            d.sub.11 = 3.4526                                                                         n.sub.06 = 1.69680                                 r.sub.12 = 1.8342                                                             ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             -0.44747 × 10.sup.-1                                                                   0.62120 × 10.sup.-2                           the 2nd lens                                                                             0.46213 × 10.sup.-2                                                                    -0.63046 × 10.sup.-2                          the 3rd lens                                                                             -0.37359 × 10.sup.-2                                                                   0.78555 × 10.sup.-3                           the 5th lens                                                                             -0.52911 × 10.sup.-2                                                                   -0.54567 × 10.sup.-4                          ______________________________________                                        f = 1               WD = 0.2780                                               m = 100 x           NA = 0.93                                                 ______________________________________                                        Embodiment 12                                                                 r.sub.1 = -0.9096                                                                            d.sub.1 = 1.3245                                                                          n.sub.01 = 1.60075*                                r.sub.2 = -1.0952                                                                            d.sub.2 = 0.0493                                               r.sub.3 = -14.1908                                                                           d.sub.3 = 1.3024                                                                          n.sub.02 = 1.49816*                                r.sub.4 = -2.1963                                                                            d.sub.4 = 0.1217                                               r.sub.5 = -10.1842                                                                           d.sub.5 = 4.1444                                                                          n.sub.03 = 1.80715*                                r.sub.6 = -39.8009                                                                           d.sub.6 = 6.9678                                               r.sub.7 = 6.1099                                                                             d.sub.7 = 4.05878                                                                         n.sub.04 = 1.69783*                                r.sub.8 = 8.5336                                                                             d.sub.8 = 3.4046                                                                          n.sub.05 = 1.72825                                 r.sub.9 = 2.0713                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             -0.45357 × 10.sup.-1                                                                   0.60265 × 10.sup.-2                           the 2nd lens                                                                             0.45552 × 10.sup.-2                                                                    -0.58051 × 10.sup.-2                          the 3rd lens                                                                             -0.78030 × 10.sup.-2                                                                   0.68623 × 10.sup.-3                           the 4th lens                                                                             -0.69381 × 10.sup.-2                                                                   -0.72030 × 10.sup.-4                          ______________________________________                                        f = 1               WD = 0.2795                                               m = 100 x           NA = 0.93                                                 ______________________________________                                        Embodiment 13                                                                 r.sub.1 = -0.9102                                                                            d.sub.1 =  1.3253                                                                         n.sub.01 = 1.60023*                                r.sub.2 = -1.0959                                                                            d.sub.2 = 0.0493                                               r.sub.3 = -14.9113                                                                           d.sub.3 = 1.3034                                                                          n.sub.02 = 1.49944*                                r.sub.4 = -2.2018                                                                            d.sub.4 = 0.1218                                               r.sub.5 = -9.1663                                                                            d.sub.5 = 4.2035                                                                          n.sub.03 = 1.85239*                                r.sub.6 = -30.0889                                                                           d.sub.6 = 6.8824                                               r.sub.7 = 5.4337                                                                             d.sub.7 = 4.0753                                                                          n.sub.04 = 1.53018*                                r.sub.8 = 5.8757                                                                             d.sub.8 = 3.4069                                                                          n.sub.05 = 1.72825                                 r.sub.9 = 1.9552                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 1st lens                                                                             -0.43188 × 10.sup.-1                                                                   0.56817 × 10.sup.-2                           the 2nd lens                                                                             0.49284 × 10.sup.-2                                                                    -0.58945 × 10.sup.-2                          the 3rd lens                                                                             -0.82238 × 10.sup.-2                                                                   0.69973 × 10.sup.-3                           the 4th lens                                                                             -0.69388 × 10.sup.- 2                                                                  -0.79032 × 10.sup.-4                          ______________________________________                                        f = 1               WD = 0.2797                                               m = 100 x           NA = 0.93                                                 ______________________________________                                        Embodiment 14                                                                 r.sub.1 = -1.0065                                                                            d.sub.1 = 1.2888                                                                          n.sub.01 = 1.70154                                 r.sub.2 = -1.1460                                                                            d.sub.2 = 0.0480                                               r.sub.3 = 42.7198                                                                            d.sub.3 = 1.2684                                                                          n.sub.02 = 1.49944*                                r.sub.4 = -2.2385                                                                            d.sub.4 = 0.1186                                               r.sub.5 = -7.2241                                                                            d.sub.5 = 4.1668                                                                          n.sub.03 = 1.85239*                                r.sub.6 = -21.1737                                                                           d.sub.6 = 3.4460                                               r.sub.7 = 4.2290                                                                             d.sub.7 = 3.9676                                                                          n.sub.04 = 1.53018*                                r.sub.8 = -11.4298                                                                           d.sub.8 = 3.3015                                                                          n.sub.05 = 1.57309                                 r.sub.9 = 1.6647                                                              ______________________________________                                                   n.sub.1        n.sub.2                                             ______________________________________                                        the 2nd lens                                                                             -0.24757 ×  10.sup.-2                                                                  -0.52109 × 10.sup.-2                          the 3rd lens                                                                             -0.51876 × 10.sup.-2                                                                   0.11213 × 10.sup.-2                           the 4th lens                                                                             -0.78173 × 10.sup.-2                                                                   -0.25718 × 10.sup.-3                          ______________________________________                                        f = 1               WD = 0.2724                                               m = 100 x           NA = 0.93                                                 ______________________________________                                    

In the above-mentioned Embodiments of the microscope objective lenssystem according to the present invention, r₁, r₂, . . . respectivelyrepresent the radii of curvature of respective lens surfaces, d₁, d₂, .. . respectively represent the thickness of respective lenses and theair-spaces between respective lenses, n₀₁, n₀₂, . . . respectivelyrepresent the refractive indices of respective lenses, or the refractiveindices on the optical axis when the lenses are GRIN lenses and in thiscase an asterisk * is given, n₁ and n₂ respectively represent the 2nd-and 4th-order coefficients, f represents the focal length of the totalsystem, WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.

In respective Embodiments of the microscope objective lens systemaccording to the present invention, the 6th- and higher-ordercoefficients are zero.

The embodiments described above have the compositions shown in FIG. 1through FIG. 14 in which a single or plural radial GRIN lenses arearranged on either the front or rear side or both the sides of theintersection between the off-axial ray l and the optical axis.

Out of the embodiments described above, the Embodiments 1 through 7 andthe Embodiments 10 through 14 comprise a meniscus lens componentarranged on the extreme object side and having a plane surface on theobject side, and has a front group of the nearly aplanatic composition.In the Embodiments 8 and 9, the lens component arranged on the extremeobject side and having a plane surface on the object side is designed asa radial GRIN lens. In the Embodiment 9, the lens groups located nearerthe object in the objective lens system have the Gauss Gauβ type ofcomposition to obtain good flatness of the image surface.

As is explained above in detail and is evident from the preferredEmbodiments, the present invention provides the microscope objectivelens system which is simpler in construction and in which aberrationsare corrected sufficiently to a wide range of NA.

I claim:
 1. A microscope objective lens system comprising a plurality oflens elements or components which are arranged along a common opticalaxis, at least one of said lens elements or components being a gradedrefractive index lens, whose refractive index varies with the radialdistance from said optical axis, and said graded refractive index lensbeing located on the object side of a point of intersection of anoff-axial principal ray and said optical axis,wherein the refractiveindex distribution of said graded refractive index lens is expressed bythe formula shown below, and said graded refractive index lens satisfiescondition (1) shown below:

    n=n.sub.0 +n.sub.1 r.sup.2 +n.sub.2 r.sup.4 +n.sub.3 r.sup.6 + . . .

    |n.sub.1 |>7×10.sup.-4 /f.sup.2    ( 1)

n₀ represents the refractive index on the optical axis, r represents theradial distance from the optical axis, n₁, n₂, n₃, . . . respectivelyrepresent the 2nd-, 4th-, 6th-, . . . order coefficients, and frepresents the focal length of said objective lens system.
 2. Amicroscope objective lens system according to claim 1, wherein a lenselement located on the most object side is a meniscus lens having aconcave surface facing the object side, and said lens element is agraded refractive index lens, whose refractive index varies with theradial distance from said optical axis.
 3. A microscope objective lenssystem according to claim 2, wherein a first positive lens located onthe image side of said meniscus lens; a second positive lens spacedapart from said first positive lens at a substantial distance; and ameniscius lens component, which has a concave surface facing the imageside; are arranged in that order from the object side, said firstpositive lens being a graded refractive index lens, whose refractiveindex varies with the radial distance from said optical axis.
 4. Amicroscope objective lens system according to claim 1, said gradedrefractive index lens further satisfying the condition (3) shown below:

    |n.sub.1 |>|n.sub.i |, i≧3 (3)

where n_(i) =n_(i) /f^(2i).
 5. A microscope objective lens systemaccording to claim 1, wherein a lens element located on the most objectside is a plano-convex lens having a plane surface on the object side,and said plano-convex lens is a graded refractive index lens whoserefractive index varies with the radial distance from said optical axis.6. A microscope objective lens system according to claim 5, wherein saidplurality of lens elements or components include first and secondpositive lens components located on the image side of said plano-convexlens element, and third lens component having a concave surface facingthe image side, said first and second positive lens components beingspaced apart from said third lens component at a substantial distance,at least one of said first and second positive lens components being agraded refractive index lens, whose refractive index varies with theradial distance from said optical axis.
 7. A microscope objective lenssystem according to claim 5, wherein said plurality of lens elements orcomponents include a pair of lens components, the facing surfaces ofwhich are both concave, on the image side of said plano-convex lenselement, said pair of lens components including at least one gradedrefractive index lens, whose refractive index varies with the radialdistance from said optical axis.
 8. A microscope objective lens systemaccording to claim 5, having the following data:

    ______________________________________                                        r.sub.1 = ∞                                                                           d.sub.1 = 0.8889                                                                           n.sub.01 = 1.63980*                                r.sub.2 = -7.4105                                                                           d.sub.2 = 0.0131                                                r.sub.3 = 1713.8486                                                                         d.sub.3 = 0.2502                                                                           n.sub.02 = 1.49700                                 r.sub.4 = -1.7788                                                                           d.sub.4 = 0.5388                                                                           n.sub.03 =1.79952*                                 r.sub.5 = -1.3234                                                                           d.sub.5 = 0.0131                                                r.sub.6 = 24.5284                                                                           d.sub.6 = 0.4578                                                                           n.sub.04 = 1.67000*                                r.sub.7 = -3.7949                                                                           d.sub.7 = 1.9044                                                r.sub.8 = 2.6069                                                                            d.sub.8 = 0.6026                                                                           n.sub.05 = 1.65830*                                r.sub.9 = ∞                                                                           d.sub.9 = 0.6903                                                                           n.sub.06 = 1.61293*                                r.sub.10 = 0.6670                                                             ______________________________________                                                    n.sub.1       n.sub.2                                             ______________________________________                                        the 1st lens                                                                              0.56264       -0.51213                                            the 3rd lens                                                                              -0.11046      0.37325 × 10.sup.-1                           the 4th lens                                                                              -0.12854      -0.73600 × 10.sup.-1                          the 5th lens                                                                              -0.25773      -0.72271 × 10.sup.-1                          the 6th lens                                                                              0.16576 × 10.sup.-2                                                                   -0.12224                                            ______________________________________                                         f = 1                                                                         m = 20                                                                        WD = 0.2427                                                                   NA = 0.46                                                                

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 9. A microscopeobjective lens system according to claims 1, 2 or 3, wherein a lenselement on the most object side is a meniscus lens having a concavesurface facing the object side, which has a uniform refractive index.10. A microscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.6144                                                                            d.sub.1 = 1.0054                                                                          n.sub.01 = 1.78590*                                r.sub.2 = -0.9063                                                                            d.sub.2 = 0.0129                                               r.sub.3 = -11.8711                                                                           d.sub.3 = 0.5453                                                                          n.sub.02 = 1.76180*                                r.sub.4 = -1.9559                                                                            d.sub.4 = 2.5506                                               r.sub.5 = 14.2613                                                                            d.sub.5 = 0.2171                                                                          n.sub.03 = 1.49700                                 r.sub.6 = -2.8644                                                                            d.sub.6 = 0.0129                                               r.sub.7 = 2.5836                                                                             d.sub.7 = 0.6761                                                                          n.sub.04 = 1.65830                                 r.sub.8 = ∞                                                                            d.sub.8 = 0.3728                                                                          n.sub.05 = 1.61340                                 r.sub.9 = 1.1081                                                              ______________________________________                                                      n.sub.1     n.sub.2                                             ______________________________________                                        the 1st lens  0.16838     0.60574 × 10.sup.-1                           the 2nd lens  -0.13761    -0.54513 × 10.sup.-2                          ______________________________________                                         f = 1                                                                         m = 20                                                                        WD =  0.2456                                                                  NA = 0.46                                                                

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 11. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.7070                                                                           d.sub.1 = 1.0402                                                                           n.sub.01 = 1.78590*                                r.sub.2 = -0.9457                                                                           d.sub.2 = 0.0130                                                r.sub.3 = -7.5963                                                                           d.sub.3 = 0.5582                                                                           n.sub.02 = 1.76180*                                r.sub.4 = -1.7635                                                                           d.sub.4 = 2.2760                                                r.sub.5 = 7.1126                                                                            d.sub.5 = 0.4596                                                                           n.sub.03 = 1.49700                                 r.sub.6 = -2.7181                                                                           d.sub.6 = 0.0130                                                r.sub.7 = 2.7615                                                                            d.sub.7 = 1.1005                                                                           n.sub.04 = 1.65830                                 r.sub.8 = 1.0582                                                              ______________________________________                                                      n.sub.1     n.sub.2                                             ______________________________________                                        the 1st lens  0.28629     0.10903                                             the 2nd lens  -0.18390    -0.78497 × 10.sup.-2                          ______________________________________                                         f = 1                                                                         m = 20                                                                        WD = 0.2486                                                                   NA = 0.46                                                                

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 12. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.4538                                                                            d.sub.1 = 0.6875                                                                          n.sub.01 = 1.83400                                 r.sub.2 = -0.7384                                                                            d.sub.2 = 0.0128                                               r.sub.3 = -2.2225                                                                            d.sub.3 = 0.2726                                                                          n.sub.02 = 1.49700                                 r.sub.4 = -1.1462                                                                            d.sub.4 = 0.0128                                               r.sub.5 = -50.2670                                                                           d.sub.5 = 0.6930                                                                          n.sub.03 = 1.76180                                 r.sub.6 = -3.5196                                                                            d.sub.6 = 2.4305                                               r.sub.7 = 3.5395                                                                             d.sub.7 = 0.3499                                                                          n.sub.04 = 1.49700                                 r.sub.8 = -4.1378                                                                            d.sub.8 = 0.0128                                               r.sub.9 = 1.9530                                                                             d.sub.9 = 0.8743                                                                          n.sub.05 = 1.65830*                                r.sub.10 = 0.9857                                                             ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 5th lens                                                                              -0.14689 × 10.sup.-2                                                                 -0.60052 × 10.sup.-3                           ______________________________________                                         f = 1                                                                         m = 20                                                                        WD = 0.2434                                                                   NA = 0.46                                                                

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 13. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.4496                                                                            d.sub.1 = 0.6812                                                                          n.sub.01 = 1.83400                                 r.sub.2 = -0.7511                                                                            d.sub.2 = 0.0127                                               r.sub.3 = -2.6794                                                                            d.sub.3 = 0.2542                                                                          n.sub.02 = 1.49700                                 r.sub.4 = -1.0686                                                                            d.sub.4 = 0.0127                                               r.sub.5 = -39.6904                                                                           d.sub.5 = 0.6420                                                                          n.sub.03 = 1.76180                                 r.sub.6 = -4.0568                                                                            d.sub.6 = 2.4876                                               r.sub.7 = 4.1051                                                                             d.sub.7 = 0.3941                                                                          n.sub.04 = 1.60178*                                r.sub.8 = -4.6364                                                                            d.sub.8 = 0.0127                                               r.sub.9 = 1.9945                                                                             d.sub.9 = 0.8333                                                                          n.sub.05 = 1.66083*                                r.sub.10 = 0.9754                                                             ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 4th lens                                                                              -0.36453 × 10.sup.-1                                                                 0.98488 × 10.sup.-2                            the 5th lens                                                                              0.13955 × 10.sup. -1                                                                 -0.25987 × 10.sup.-1                           ______________________________________                                         f = 1                                                                         m = 20                                                                        WD = 0.2427                                                                   NA = 0.46                                                                

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 14. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.6202                                                                           d.sub.1 = 1.0101                                                                           n.sub.01 = 1.78590*                                r.sub.2 = -0.9334                                                                           d.sub.2 = 0.0129                                                r.sub.3 = -25.9860                                                                          d.sub.3 = 0.5460                                                                           n.sub.02 = 1.76180*                                r.sub.4 = -1.8355                                                                           d.sub.4 = 2.5837                                                r.sub.5 = 17.0878                                                                           d.sub.5 = 0.2177                                                                           n.sub.03 = 1.49700                                 r.sub.6 = -2.8475                                                                           d.sub.6 = 0.0129                                                r.sub.7 = 2.3126                                                                            d.sub.7 = 1.0278                                                                           n.sub.04 = 1.65830*                                r.sub.8 = 1.3126                                                              ______________________________________                                                     n.sub.1       n.sub.2                                            ______________________________________                                        the 1st lens 0.20954       0.55868 × 10.sup.-1                          the 2nd lens -0.12844      0.81108 × 10.sup.-2                          the 4th lens 0.18233 × 10.sup.-2                                                                   0.56092 × 10.sup.-3                          ______________________________________                                         f = 1                                                                         m = 20                                                                        WD = 0.2464                                                                   NA =  0.46                                                               

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 15. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.6082                                                                           d.sub.1 = 0.9974                                                                           n.sub.01 = 1.77250*                                r.sub.2 = -0.9962                                                                           d.sub.2 = 0.0127                                                r.sub.3 = 4.5931                                                                            d.sub.3 = 0.5346                                                                           n.sub.02 = 1.77250*                                r.sub.4 = -2.1599                                                                           d.sub.4 = 2.6874                                                r.sub.5 = 5.1919                                                                            d.sub.5 = 1.0629                                                                           n.sub.03 = 1.83400*                                r.sub.6 = 1.5327                                                              ______________________________________                                                     n.sub.1      n.sub.2                                             ______________________________________                                        the 1st lens 0.15622      0.27687 × 10.sup.-1                           the 2nd lens -0.93618 × 10.sup.-1                                                                 0.19220 × 10.sup.-1                           the 3rd lens -0.16188     0.32660 × 10.sup.-1                           ______________________________________                                         f = 1                                                                         m = 20                                                                        WD = 0.2417                                                                   NA = 0.46                                                                

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 16. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.5249                                                                            d.sub.1 = 0.9910                                                                          n.sub.01 = 1.77250                                 r.sub.2 = -1.0014                                                                            d.sub.2 = 0.0124                                               r.sub.3 = 4.7627                                                                             d.sub.3 = 0.5335                                                                          n.sub.02 = 1.77250*                                r.sub.4 = -2.2014                                                                            d.sub.4 = 2.6257                                               r.sub.5 = -23.3838                                                                           d.sub.5 = 1.0309                                                                          n.sub.03 = 1.83400*                                r.sub.6 = 3.3579                                                              ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 2nd lens                                                                              -0.55298 × 10.sup.-1                                                                 -0.63272 × 10.sup.-2                           the 3rd lens                                                                              -0.13406     0.43943 × 10.sup.-1                            ______________________________________                                        f = 1               WD = 0.2371                                               m = 20 ×      NA = 0.46                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 17. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.8938                                                                           d.sub.1 = 1.3014                                                                           n.sub.01 = 1.60311*                                r.sub.2 = -1.0761                                                                           d.sub.2 = 0.0485                                                r.sub.3 = -12.5422                                                                          d.sub.3 = 1.2779                                                                           n.sub.02 = 1.49700*                                r.sub.4 = -2.1073                                                                           d.sub.4 = 0.1196                                                r.sub.5 = -5.9203                                                                           d.sub.5 = 4.0147                                                                           n.sub.03 = 1.65830*                                r.sub.6 = -15.8260                                                                          d.sub.6 = 2.4285                                                r.sub.7 = 50.5226                                                                           d.sub.7 = 1.2080                                                                           n.sub.04 = 1.69680                                 r.sub.8 = -45.6551                                                                          d.sub.8 = 0.8486                                                r.sub.9 = 8.8504                                                                            d.sub.9 = 3.9661                                                                           n.sub.05 = 1.64769*                                r.sub.10 = 8.1084                                                                           d.sub.10 = 0.0901                                               r.sub.11 = 5.3751                                                                           d.sub.11 = 3.3167                                                                          n.sub.06 = 1.69680*                                r.sub.12 = 1.8122                                                             ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 1st lens                                                                              -0.55383 × 10.sup.-1                                                                 0.75335 × 10.sup.-2                            the 2nd lens                                                                              0.36158 × 10.sup.-2                                                                  -0.67351 × 10.sup.-2                           the 3rd lens                                                                              -0.48852 × 10.sup.-2                                                                 0.55867 × 10.sup.-3                            the 5th lens                                                                              -0.91403 × 10.sup.-2                                                                 0.61851 × 10.sup.-4                            the 6th lens                                                                              0.29971 × 10.sup.-2                                                                  -0.79363 × 10.sup.-3                           ______________________________________                                        f = 1               WD = 0.2747                                               m = 100 ×     NA = 0.93                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 18. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.9046                                                                           d.sub.1 = 1.3174                                                                           n.sub.01 = 1.60311*                                r.sub.2 = -1.0892                                                                           d.sub.2 = 0.0490                                                r.sub.3 = -14.5015                                                                          d.sub.3 = 1.2957                                                                           n.sub.02 = 1.49700*                                r.sub.4 = -2.1711                                                                           d.sub.4 = 0.1210                                                r.sub.5 = -9.4617                                                                           d.sub.5 = 4.1552                                                                           n.sub.03 = 1.65830*                                r.sub.6 = -74.3330                                                                          d.sub.6 = 2.6622                                                r.sub.7 = 14.4729                                                                           d.sub.7 = 1.2227                                                                           n.sub.04 = 1.69680                                 r.sub.8 = 76.2347                                                                           d.sub.8 = 1.4766                                                r.sub.9 = 7.4458                                                                            d.sub.9 = 4.4476                                                                           n.sub.05 = 1.64769*                                r.sub.10 = 5.3166                                                                           d.sub.10 = 0.0912                                               r.sub.11 = 4.1294                                                                           d.sub.11 = 3.4526                                                                          n.sub.06 = 1.69680                                 r.sub.12 = 1.8342                                                             ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 1st lens                                                                              -0.44747 × 10.sup.-1                                                                 0.62120 × 10.sup.-2                            the 2nd lens                                                                              0.46213 × 10.sup.-2                                                                  -0.63046 × 10.sup.-2                           the 3rd lens                                                                              -0.37359 × 10.sup.-2                                                                 0.78555 × 10.sup.-3                            the 5th lens                                                                              -0.52911 × 10.sup.-2                                                                 -0.54567 × 10.sup.-4                           ______________________________________                                        f = 1               WD = 0.2780                                               m = 100 ×     NA = 0.93                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 19. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.9096                                                                            d.sub.1 = 1.3245                                                                          n.sub.01 = 1.60075*                                r.sub.2 = -1.0952                                                                            d.sub.2 = 0.0493                                               r.sub.3 = -14.1908                                                                           d.sub.3 = 1.3024                                                                          n.sub.02 = 1.49816*                                r.sub.4 = -2.1963                                                                            d.sub.4 = 0.1217                                               r.sub.5 = -10.1842                                                                           d.sub.5 = 4.1444                                                                          n.sub.03 = 1.80715*                                r.sub.6 = -39.8009                                                                           d.sub.6 = 6.9678                                               r.sub.7 = 6.1099                                                                             d.sub.7 = 4.0578                                                                          n.sub.04 = 1.69783*                                r.sub.8 = 8.5336                                                                             d.sub.8 = 3.4046                                                                          n.sub.05 = 1.72825                                 r.sub.9 = 2.0713                                                              ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 1st lens                                                                              -0.45357 × 10.sup.-1                                                                 0.60265 × 10.sup.-2                            the 2nd lens                                                                              0.45552 × 10.sup.-2                                                                  -0.58051 × 10.sup.-2                           the 3rd lens                                                                              -0.78030 ×  10.sup.-2                                                                0.68623 × 10.sup.-3                            the 4th lens                                                                              -0.69381 × 10.sup.-2                                                                 -0.72030 × 10.sup.-4                           ______________________________________                                        f = 1               WD = 0.2795                                               m = 100 ×     NA = 0.93                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 20. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -0.9102                                                                            d.sub.1 = 1.3253                                                                          n.sub.01 = 1.60023*                                r.sub.2 = -1.0959                                                                            d.sub.2 = 0.0493                                               r.sub.3 = -14.9113                                                                           d.sub.3 = 1.3034                                                                          n.sub.02 = 1.49944*                                r.sub.4 = -2.2018                                                                            d.sub.4 = 0.1218                                               r.sub.5 = -9.1663                                                                            d.sub.5 = 4.2035                                                                          n.sub.03 = 1.85239*                                r.sub.6 = -30.0889                                                                           d.sub.6 = 6.8824                                               r.sub.7 = 5.4337                                                                             d.sub.7 = 4.0753                                                                          n.sub.04 = 1.53018*                                r.sub.8 = 5.8757                                                                             d.sub.8 = 3.4069                                                                          n.sub.05 = 1.72825                                 r.sub.9 = 1.9552                                                              ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 1st lens                                                                              -0.43188 × 10.sup.-1                                                                 0.56817 × 10.sup.-2                            the 2nd lens                                                                              0.49284 × 10.sup.-2                                                                  -0.58945 × 10.sup.-2                           the 3rd lens                                                                              -0.82238 × 10.sup. -2                                                                0.69973 × 10.sup.-3                            the 4th lens                                                                              -0.69388 × 10.sup.-2                                                                 -0.79032 × 10.sup.-4                           ______________________________________                                        f = 1               WD = 0.2797                                               m = 100 ×     NA = 0.93                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 21. Amicroscope objective lens system according to claim 9, having thefollowing data:

    ______________________________________                                        r.sub.1 = -1.0065                                                                            d.sub.1 = 1.2888                                                                          n.sub.01 = 1.70154                                 r.sub.2 = -1.1460                                                                            d.sub.2 = 0.0480                                               r.sub.3 = 42.7198                                                                            d.sub.3 = 1.2684                                                                          n.sub.02 = 1.49944*                                r.sub.4 = -2.2385                                                                            d.sub.4 = 0.1186                                               r.sub.5 = -7.2241                                                                            d.sub.5 = 4.1668                                                                          n.sub.03 = 1.85239*                                r.sub.6 = -21.1737                                                                           d.sub.6 = 3.4460                                               r.sub.7 = 4.2290                                                                             d.sub.7 = 3.9676                                                                          n.sub.04 = 1.53018*                                r.sub.8 = -11.4298                                                                           d.sub.8 = 3.3015                                                                          n.sub.05 = 1.57309                                 r.sub.9 = 1.6647                                                              ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 2nd lens                                                                              -0.24757 × 10.sup.-2                                                                 -0.52109 × 10.sup.-2                           the 3rd lens                                                                              -0.51876 × 10.sup.-2                                                                 0.11213 × 10.sup.-2                            the 4th lens                                                                              -0.78173 × 10.sup.-2                                                                 -0.25718 × 10.sup.-3                           ______________________________________                                        f = 1               WD = 0.2724                                               m = 100 ×     NA = 0.93                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.
 22. Amicroscope objective lens system according to claim 9, including a firstpositive lens component, second positive lens component, and a thirdlens component having a concave surface facing the image side, in thatorder, at the rear of said meniscus lens, said first positive lenscomponent being spaced apart from said second positive lens component ata substantial distance, and each of said meniscus lens and said firstpositive lens component being a graded refractive index lens, whoserefractive index varies with the radial distance from the optical axis.23. A microscope objective lens system according to claim 22, whereinsaid third lens component, which has a concave surface facing the imageside, is a graded refractive index lens, whose refractive index varieswith the radial distance from the optical axis.
 24. A microscopeobjective lens system according to claim 9, including a first positivelens component and second lens component having a concave surface facingthe image side, in that order, at the rear of said meniscus lens, saidfirst positive lens component being spaced apart from said second lenscomponent at a substantial distance and both said first positive lenscomponent and said second lens component being a graded refractive indexlens, whose refractive index vary with the radial distance from theoptical axis.
 25. A microscope objective lens system according to claim24, wherein said meniscus lens is a graded refractive index lens, whoserefractive index varies with the radial distance from the optical axis.26. A microscope objective lens system according to claim 9, includingfirst and second lens components and third lens component having aconcave surface facing the image side, in that order, at the rear ofsaid meniscus lens, said second lens component being spaced apart fromsaid third lens component at a substantial distance at least one ofthese lens components being a graded refractive index lens, whoserefractive index varies with the radial distance form the optical axis.27. A microscope objective lens system according to claim 26, whereinsaid meniscus lens is a graded refractive index lens, whose refractiveindex varies with the radial distance from the optical axis.
 28. Amicroscope objective lens system according to claim 27, wherein saidthird lens component is a cemented doublet.
 29. A microscope objectivelens system according to claim 27, wherein said third lens componentconsists of two lens elements separated from each other.
 30. Amicroscope objective lens system according to claim 27, wherein a fourthlens component is arranged between said second lens component and saidthird lens component.
 31. A microscope objective lens system accordingto claim 30 wherein said third lens component is a cemented doublet. 32.A microscope objective lens system according to claim 30 wherein saidthird lens component consists of two lens elements separated from eachother.
 33. A microscope objective lens system comprising a plurality oflens elements or components which are arranged along a common opticalaxis, at least one of said lens elements or components being a gradedrefractive index lens, whose refractive index varies with the radialdistance from the optical axis, and said graded refractive index lensbeing located on the image side of a point of intersection of anoff-axial principal ray and said optical axis, wherein the refractiveindex distribution of said graded refractive index lens is expressed bythe formula shown below and said graded refractive index lens satisfiesthe condition (2) shown below.

    n=n.sub.0 +n.sub.1 r.sup.2 +n.sub.2 r.sup.4 +n.sub.3 r.sup.6 + . . .

    |n.sub.1 |>7×10.sup.-4 /f.sup.2    ( 2)

where n₀ represents the refractive index on the optical axis, rrepresents the radial distance from the optical axis, n₁, n₂, n₃, . . .respectively represent the 2nd-, 4th-, 6th-, . . . order coefficients,and f represents the focal length of said objective lens system.
 34. Amicroscope objective lens system according to claim 33, wherein a lenscomponent located on the most image side has a concave surface facingthe image side, and said lens component comprises at least one gradedrefractive index lens, whose refractive index varies with the radialdistance from said optical axis.
 35. A microscope objective lens systemaccording to claim 34, wherein a meniscus lens component having aconcave surface facing the object side, and first, second and third,positive lenses are located in that order on the object side, saidsecond positive lens being spaced apart from said third positive lens ata distance.
 36. A microscope objective lens system according to claim 35wherein said third positive lens is a graded refractive index lens,whose refractive index varies with the radial distance from said opticalaxis.
 37. A microscope objective lens system according to claim 33, saidgraded refractive index lens further satisfying the condition (3) shownbelow:

    |n.sub.1 |>|n.sub.i |, i≧3 (3)

where n_(i) =n_(i) /f^(2i).
 38. A microscope objective lens systemcomprising a plurality of lens elements or components which are arrangedalong a common optical axis, at least two of said lens elements orcomponents being graded refractive index lenses whose refractive indexvary with the radial distance from the optical axis, at least one ofsaid graded refractive index lenses being located on the object side ofa point of intersection of an off-axial principal ray and said opticalaxis, and the remaining graded refractive index lenses being located onthe image side of said point,wherein the refractive index distributionof said graded refractive index lenses is expressed by the formula shownbelow, said graded refractive index lens on the object side of saidpoint satisfies the condition (1) shown below, and said gradedrefractive index lens on the image side of said point satisfies thecondition (2) shown below:

    n=n.sub.0 +n.sub.1 r.sup.2 +n.sub.2 r.sup.4 +n.sub.3 r.sup.6 + . . .

    |n.sub.1 |>7×10.sup.-4 /f.sup.2    ( 1)

    |n.sub.1 |>1×10.sup.-4 /f.sup.2    ( 2)

n₀ represents the refractive index on the optical axis, r represents theradical distance from the optical axis, n₁, n₂, n₃, . . . respectivelyrepresent the 2nd-, 4th-, 6th-, . . . order coefficients, and frepresents the focal length of said objective lens system.
 39. Amicroscope objective lens system according to claim 38, wherein saidplurality of lens elements or components include a foremost meniscuslens element located on the most object side, said meniscus lens elementhaving a concave surface facing the object side, and a rearmost lenscomponent located on the most image side, said lens component having aconcave surface facing the image side, and said meniscus lens elementbeing a graded refractive index lens, whose refractive index varies withthe radial distance from said optical axis.
 40. A microscope objectivelens system according to claim 39, further including a positive lens,and a thick lens having a concave surface facing the object side saidrearmost lens components consisting of two lens elements, said thicklens and either of said two lens elements of said rearmost lenscomponent being graded refractive index lenses, whose refractive indicesvary with the radial distances from the optical axis.
 41. A microscopeobjective lens system according to claim 40, wherein another lenscomponent is located between said thick lens and said rearmost lenscomponent.
 42. A microscope objective lens system according to claim 41,wherein both of said two lens elements of said rearmost lens componentare graded refractive index lenses, whose refractive indices vary withthe radial distances from the optical axis.
 43. A microscope objectivelens system according to claim 38, wherein said plurality of lenselements or components include a meniscus lens element located on themost object side, said meniscus lens element having a concave surfacefacing the object side, and a rearmost lens component located on themost image side, said rearmost lens component having a concave surfacefacing the image side, and said rearmost lens component including atleast one graded refractive index lens, whose refractive index varieswith the radial distance from said optical axis.
 44. A microscopeobjective lens system according to claim 43, wherein a first positivelens element is located on the image side of said meniscus lens element,said positive lens element being a graded refractive index lens, whoserefractive index varies with the radial distance from the optical axis.45. A microscope objective lens system according to claim 44, wherein asecond lens element is located on the image side of said first positivelens element, and has a concave surface facing the object side, saidsecond lens element being a graded refractive index lens, whoserefractive index varies with the radial distance from the optical axis.46. A microscope objective lens system according to claim 38, whereinsaid plurality of lens elements or components include a meniscus lenselement located on the most object side, said meniscus lens elementhaving a concave surface facing the object side, and a rearmost lenscomponent located on the most image side, said rearmost lens componenthaving a concave surface facing the image side, said meniscus lenselement being a graded refractive index lens, and said rearmost lenscomponent including at least one graded refractive index lenses, therefractive index of each of said graded refractive index lenses varyingwith the radial distances from said optical axis.
 47. A microscopeobjective lens system according to claim 46, wherein a first positivelens is located on the image side of said meniscus lens element, saidfirst positive lens being a graded refractive index lens, whoserefractive index varies with the radial distance from said optical axis.48. A microscope objective lens system according to claim 47, wherein asecond positive lens is located on the object side of said rearmost lenscomponent.
 49. A microscope objective lens system according to claim 38,said graded refractive index lens further satisfying the condition (3)shown below:

    |n.sub.1 |>|n.sub.i |, i≧3 (3)

where n_(i) =n_(i) /f^(2i).
 50. A microscope objective lens systemaccording to claim 38, having the following data:

    ______________________________________                                        r.sub.1 = ∞                                                                           d.sub.1 = 0.5027                                                                           n.sub.01 = 1.83400*                                r.sub.2 = -0.7363                                                                           d.sub.2 = 0.0125                                                r.sub.3 = 0.5038                                                                            d.sub.3 = 0.2311                                                                           n.sub.02 = 1.49700                                 r.sub.4 = ∞                                                                           d.sub.4 = 0.1868                                                                           n.sub.03 = 1.74000*                                r.sub.5 = 0.3873                                                                            d.sub.5 = 0.2353                                                r.sub.6 = -0.3567                                                                           d.sub.6 = 0.1005                                                                           n.sub.04 = 1.67270*                                r.sub.7 = ∞                                                                           d.sub.7 = 0.4094                                                                           n.sub.05 = 1.49700*                                r.sub.8 = -0.7164                                                                           d.sub.8 = 0.0184                                                r.sub.9 = 4.7830                                                                            d.sub.9 = 0.3580                                                                           n.sub.06 = 1.66755*                                r.sub.10 = -1.2326                                                                          d.sub.10 = 1.5374                                               r.sub.11 = 1.0611                                                                           d.sub.11 = 0.6214                                                                          n.sub.07 = 1.54869                                 r.sub. 12 = 0.7628                                                            ______________________________________                                                    n.sub.1      n.sub.2                                              ______________________________________                                        the 1st lens                                                                              -0.24460     -0.25363                                             the 3rd lens                                                                              0.36245 × 10.sup.-1                                                                  0.48691                                              the 4th lens                                                                              0.92425 × 10.sup.-1                                                                  0.63217                                              the 5th lens                                                                              0.10171      0.75023 × 10.sup.-1                            the 6th lens                                                                              -0.58875 × 10.sup.-1                                                                 -0.22649 × 10.sup.-2                           ______________________________________                                        f = 1               WD = 0.4513                                               m = 20 ×      NA = 0.46                                                 ______________________________________                                    

where r₁, r₂, . . . respectively represent the radii of curvature ofrespective lens surfaces, d₁, d₂, . . . respectively represent thethickness of respective lenses and the air-spaces between respectivelenses, n₀₁, n₀₂, . . . respectively represent the refractive indices ofrespective lenses (or the refractive indices on the optical axis in thecase of said graded refractive index lenses to which an asterisk * isattached), WD represents the working distance, m represents themagnification, and NA represents the numerical aperture.